The present invention relates to an apparatus and a method for reshaping tubes, in particular helically-wound lock-seam tubes for ventilation ductwork.
WO-A-99/51371 discloses an apparatus, referred to as ovalizer, for forming round ductwork to oval, that is an apparatus for altering the cross-section of a tube from round to oval. In particular, the tube is a spiral duct, also referred to as helically-wound tubing by the skilled person.
To achieve this reshaping of the tube, the apparatus has an elongate, horizontal duct forming assembly with two duct forming members vertically spaced from each other. The two duct forming members are connected to each other by power means configured to move the duct forming members to and from each other vertically.
A round tube to be reshaped is placed on the duct forming assembly and the power means (several hydraulic cylinders) is activated. As the distance between the duct forming members is increased, they press against the inner surface of the tube in two opposite locations and the tube is reshaped to an oval cross-section. Each duct forming member has a semi-circular head which presses against the inner surface of the tube.
This known technique has, however, some drawbacks which will be discussed below.
First, due to the pressing action against the inside of the tube, the tube wall is deformed and the material is stretched. This stretching may lead to ruptures and other defects in the tube wall.
Second, when the tube is a helically-wound lock-seam tube (normal case), the pressing action may lead to slippage or sliding in the lock seam, which in turn may produce tubes with different dimensions at each end. This makes it difficult to connect the tubes in a ventilation duct system.
Third, the semi-circular heads of the duct forming members have to be replaced for each diameter of the tube. The replacement operation is time-consuming, and several duct forming heads have to be kept in stock. This adds costs.
Fourth, the known apparatus has low flexibility since it can only be used for reshaping from round to oval.
Fifth, the deformed or reshaped tube (oval) tends to return to its original shape (round) due to stresses in the tube wall induced by the deformation.
Thus, there is a demand for an improved technique for reshaping of tubes.
As to background art, the so-called bending machines should be mentioned as well. A bending machine has a roller assembly to which a plate or sheet of metal is fed and formed to the desired shape, for instance to round cross-section. In a final step, the tube is closed by a longitudinal weld along the tube wall. However, these bending machines cannot be used for reshaping of a tube from, for instance, round to oval since the starting material always is a flat plate or sheet.
An object of the present invention is to provide a new and improved technique for reshaping of tubes, in particular helically-wound lock-seam tubes for ventilation ductwork, thereby avoiding or at least making the disadvantages discussed above less severe.
This object is now achieved by an apparatus for reshaping a tube, the tube having a wall with a periphery, an inner surface, an outer surface, a longitudinal axis and a cross-section having at least one radius of curvature, comprising:
means configured to rotate the tube about the longitudinal axis in the apparatus;
a first shaping roller parallel with the longitudinal axis of the tube and configured to engage the outer surface of the tube;
a second shaping roller parallel with the longitudinal axis of the tube and configured to engage the inner surface of the tube; and
means configured to move the first and second shaping rollers in relation to each other and in relation to the rotation means, in order to apply bending forces to the tube wall for altering said at least one radius of curvature and thereby reshaping the cross-section of the tube.
By this apparatus, the problems of the prior art technique are solved. Thanks to the ingenious arrangement of the shaping rollers and the means for moving these, it is very easy to reshape the tube. The material of the tube is not stretched since the circumference of the tube is not changed by the reshaping operation. Hence, no defects occur in the tube wall.
Furthermore, the apparatus can be used for reshaping a great number of tubes, from round to oval, from oval to round, etc. In principle, any cross-sectional shape of the tube is possible to achieve, as long as it has rounded corners, for instance triangular, rectangular, etc. Thus, the operation of the inventive apparatus is very flexible.
Another very important advantage is that the same apparatus can be used for all dimensions. No tube forming members have to be replaced when tubes of various dimensions are to be reshaped. The same roller arrangement can be used for all dimensions. Indeed, this is advantageous in comparison with the apparatus known from WO-A-99/51371 discussed by way of introduction.
In a preferred embodiment, the shaping rollers are configured to apply the bending forces in points of action which are spaced with respect to the periphery of the wall. As a result, a very smooth reshaping operation can be achieved. Preferably, the shaping rollers are configured to apply the bending forces along generatrices of the tube, which further enhances the reshaping operation.
It is preferred that the rotation means comprises a roller nip which has two opposite rollers configured to engage the tube wall in a nip and drivingly rotate the tube along its longitudinal axis. Such a roller nip provides for a secure and smooth rotational operation. Most preferably, the shaping rollers and the nip rollers are parallel in order to provide for simple control of the reshaping operation. Furthermore, the structure of the apparatus is very compact by this arrangement.
In a preferred embodiment, the apparatus comprises a frame with two opposite, spaced frame members between which the tube is disposed in operation. Each frame member has guiding means to which end portions of the shaping rollers are connected and which are configured to provide said movement of the shaping rollers. These guiding means in the frame members provide for a very distinct control of the reshaping movement of the shaping rollers. No disadvantageous inertia is involved.
Preferably, the guiding means are grooves provided in the respective frame members. Furthermore, it is preferred that actuator means, such as hydraulic cylinders, are mounted between the end portions of the shaping rollers and the frame members. Consequently, a distinct movement of the shaping rollers is achieved.
In one aspect of the invention, there is provided a roller unit for reshaping a tube having a cross-section and a longitudinal axis, comprising:
a first roller assembly configured to rotate the tube about the longitudinal axis,
a second roller assembly configured to apply bending forces to the tube directed towards or away from the tube, thereby altering the cross-section of the tube.
The object is also achieved by a method of reshaping a tube, the tube having an inner surface, an outer surface, a longitudinal axis and a cross-section having at least one radius of curvature, comprising the steps of:
rotating the tube about the longitudinal axis; and
applying bending forces on the inner and outer surfaces of the tube;
hereby altering said at least one radius of curvature for reshaping the cross-section of the tube.
The advantages of the method of the invention, and its preferred variants, are basically the same as discussed above in relation to the apparatus.